| Year | Value | | 2017 | 1.47 | | 2016 | 1.4 | | 2015 | 1.38 | | 2014 | 1.89 | | 2013 | 1.8 | | 2012 | 1.8 | | 2011 | 1.8 | | 2010 | 1.8 | | 2009 | 1.8 | | 2008 | 1.8 | | 2007 | 1.9 | | 2006 | 1.9 | | 2005 | 1.7 | | 2004 | 1.7 | | 2003 | 1.7 | | 2002 | 1.7 | | 2001 | 1.7 | | 2000 | 1.7 | | 1997 | 1.7999999523 | | 1994 | 1.6302000284 | | 1990 | 1.7977999449 | | 1985 | 1.2055000066757 | | 1981 | 1.2063000202179 | | 1980 | 1.2576999664307 | | 1975 | 1.0260000228882 | | 1970 | 1.1280000209808 | | 1960 | 3.6054501533508 |
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